1. Field of the Invention
This invention relates to a semiconductor device such as a power transistor and a power semiconductor IC, and more particularly to a semiconductor device having a heat radiating portion partially exposed from a mold package.
2. Description of the Related Art
FIG. 1 is a perspective view showing the construction of conventional power IC device 30 as viewed from the rear side. Power IC device 30 includes semiconductor chip CP, mold resin 31, heat radiating metal plate 33 and lead terminals 34. Heat radiating plate 33 includes island portion 32. Semiconductor chip CP is fixed on island portion 32 on the front surface of heat radiating plate 33, and lead terminals 34 are connected to semiconductor chip CP. Further, semiconductor chip CP is packaged together with lead terminal 34 and heat radiating plate 33 by mold resin 31, and part of the rear surface of heat radiating plate 33 is exposed. The power IC device further includes hole 35 formed through heat radiating plate 33 and mold resin 31.
In order to mount IC device 30 on heat radiation chassis 41 of an electronic instrument, transistor device 30 is fixed to chassis 41 by means of screw 42 or clip as shown in FIGS. 2A and 2B and at the same time the exposed portions of lead terminals 34 are connected to the circuit of the electronic instrument. More specifically, IC device 30 is mounted with the rear surface of heat radiation plate 33 held to face heat radiation chassis 41, and lead terminals 34 are soldered to printing circuit board 43 of the circuit of the electronic instrument.
In general, IC device 30 will be curved after the molding process as shown in FIG. 2C because of the difference between the heat characteristics of the mold resin and metal of the heat radiation plate. When the IC device is mounted by screw 42 or clip in the condition shown in FIGS. 2C, it will be less curved. However, at this time, cracks will be easily made in the semiconductor chip by the mechanical stress. Further, if temperature of transistor device 30 rises during the operation, the mold resin is deformed by thermal stress and cracks may be made in the semiconductor chip, particularly when mold resin 31, heat radiation plate 33 and semiconductor chip CP are of low-quality, materials. Therefore, it is difficult to reduce the possibility of making a chip having cracks to 0.1% or less by using the ordinary screening method in the mass-production process.
In order to prevent cracks from being made in the semiconductor chip, materials having expansion coefficients substantially equal to each other may be used to form a less curved IC device, for example. However, in this case, it becomes difficult to maintain the present heat radiation efficiency of IC device 30. For this reason, it is now common practice to set optimum margin for formation of the cracks by strictly controlling the manufacturing process of IC device 30